From my (only) 40 hours or so experience so far, the diplomacy screen doesn't have that option until certain things happen - like you capturing a city. I'm guessing this is somewhat similar to how the AI in 5 would say stuff like "Whats the matter? Tired of battle?" for a while when you first went to war until they'd lose units or cities. I've had this same scenario happen every time I got to war in Civ 6 so far. The only sure fire way I've personally seen to force peace is to directly threaten your enemy's capital, then suddenly they offer all their junk for peace.
